JP-10-331734A shows a direct injection engine which is provided with a high-pressure pump. The high-pressure pump is driven by the engine to pressurize and atomize the fuel which is pumped up from a fuel tank by a low-pressure pump.
As shown by a dashed line in FIG. 3, the pressure of the fuel in a high-pressure pipe between the high-pressure pump and the fuel injector increases for a certain period after engine is shut down as a temperature of the engine increases due to residual heat of the engine. After the certain period has passed, the fuel pressure decreases as the temperature of the fuel decreases due to a natural radiation of heat from the engine. For instance, in a direct injection engine, since the fuel pressure at idle right before the engine is shut down (i.e. when engine shut down is imminent) remains high (for example, 8 MPa), the time period in which the fuel pressure remains high after the engine is shut down is prolonged. Furthermore, as shown in FIG. 4, as the fuel pressure increases while the engine is stopped, fuel leakage from the fuel injector is more likely. The leaked fuel may remain in the cylinder and may be expelled as unburned fuel, which may cause undesirable emissions during the next start of the engine.
JP-2004-232494 shows an intake port injection engine, which is provided with a fuel return pipe for returning the fuel in the fuel pipe to the fuel tank. The return pipe is provided with an orifice to reduce the fuel pressure by returning the fuel in the fuel pipe to the fuel tank through the orifice after the engine is shut down.
If such a return pipe having the orifice is applied to the direct injection engine, since the fuel is rapidly depressurized from high pressure to atmospheric pressure when passing through the orifice, a fuel vapor may be crated in the fuel returning to the fuel tank. It may cause a vapor lock at the next starting of the engine.